The subject invention relates to magnetic recording systems and particularly to "head-servo subsystems" therein, i.e., systems adapted to position transducers radially across a recording disk and thus register it with a selected track thereon.
Workers in the magnetic recording arts are aware that there is a need for improved techniques and associated apparatus for properly registering transducer means relative to any selected data track on a magnetic record. Efforts have continued for sometime now to fill this need.
One such recording system involves a magnetic disk typically used as peripheral memory equipment in a computer system to provide (temporary or permanent) information storage during computer operations. In one well-known configuration, one or several such disks are mounted to be rapidly rotated in operative relation with transducer means, each disk having a multi-track magnetic recording surface on at least one of its faces. One, or several, recording heads are, in turn, adapted to be positioned along each disk-face--i.e., to register with any selected one of the concentric recording tracks on the disk to record and detect data signals along any track.
Workers will recognize that optimal use of such media requires that information be recorded at the highest possible density. Present-day techniques record at densities of up to thousands--in certain cases tens of thousands--of bits per inch. Similarly, workers strive to maximize the number of circumferential tracks on a disk, with each track as narrow as possible. Accordingly, with ever-higher bit densities and track densities, it is apparent that head-positioning systems are being pushed to their limit. Systems for quickly and accurately registering heads with a selected data track are becoming more and more sophisticated, and more complex and expensive, and their operating parameters more stringent. The invention responds to this need, teaching a novel technique whereby "track-on-data" is feasible.
Workers will recognize that for the typical magnetic disk system a recording head is translated radially across the disk so that the magnetic transducer gaps mounted therein may be selectively positioned adjacent a selected recording track. In this way only a few transducer gaps need be used for recording and reading data on a number of disk tracks--but to practically implement such a system, a very careful, accurate control of head location relative to the tracks must be kept--and this typically must be done very quickly to minimize access time for the computer system served.
For instance, with disks used in a random access magnetic memory the data bits are recorded in concentric circular tracks so there is a continual need to secure and maintain very accurate registry of a magnetic transducer with a selected track. Unfortunately, the precision of the transducer-positioning system will determine track spacing tolerances and accordingly will influence data storage efficiency (bit compression) significantly--that is the number of characters per unit memory area will depend upon the accuracy of transducer positioning. Workers have attempted in various ways to improve the accuracy of transducer positioning, for "servoing" the transducer onto disk tracks. Such systems have commonly employed "position signal" tracks (or track sectors) interspersed with the data tracks and have also required a special servo transducer detector to detect the "position-signals". They also add the operation of writing the servo data. Such features inherently degrade data storage efficiency--because of the separate servo transducer system required (e.g., buildup of mechanical tolerances in the different transducers used; because of the considerable loss of useful data recording area to recording position-bits, and so forth).
As workers know (also see U.S. Pat. Nos. 3,691,5431; 3,812,533; and 3,838,457) "fine" positioning is typically achieved by controlling the movement of a head positioning carriage in response to the detection of pre-recorded encoded servo data, using either the data transducer or a special servo transducer. The servo data is either recorded on the same disk as the work data or else on a separate disk, or on a like surface having a precise mechanical relationship with the work data surface.
"Coarse" positioning is typically achieved in either of two ways: (1) by controlling radial head movement based on detection of the movement of the head positioning carriage, such as by photo-electric detection means (e.g., see U.S. Pat. No. 3,812,533), or (2) by controlling radial head movement based on detected track crossings, while using recorded servo data for fine positioning purposes (e.g., see U.S. Pat. Nos. 3,691,543, and 3,838,457).
Workers are also aware of various positioning techniques (using mechanical, hydraulic and/or electro-magnetic means) for registering magnetic transducers with associated recording tracks. Certain noteworthy mechanical or optical techniques are known for monitoring transducer position relative to a track and providing feedback signals which may be used to control a servo positioning-control system adapted to keep the transducer gap(s) registered with the track. However, none of the present known techniques for monitoring and controlling head position is wholly satisfactory--partly because of the extensive complex, auxiliary equipment they require and/or because of the limitation these approaches place on track density. An object of this invention is to provide an answer to this problem by teaching improved "track-on-data" techniques and associated systems.
Workers in these arts will recognize that it is quite desirable to "track-on-data", that is to somehow use the area devoted to "data-bits" (i.e., "information signals" developed from certain magnetic transitions) to also provide position control signals which may be fed to a positioning servo and control the positioning and/or alignment of a transducer relative to a recorded track. Obviously, such a technique can eliminate the need for a separate "servo" recording unit and related separate recording zones for servo data (such as separate servo disks or separate servo tracks, or track-sectors, typically seen in conventional magnetic recording systems) since the data-transducer and the data-recording zones may be used for servo-bits too. The invention accomplishes this, providing a "track-on data" system with no need for separate servo tracks and providing a "servo transducer" which is integrated with a "data transducer", with servo-bits being incorporated into the data recording zones as desired.
Workers will recognize the significant advantages from such a "track-on-data" technique. For instance, present-day magnetic disk memory systems typically allocate servo-bits to special "servo tracks" (either on a special portion of each disk or on a special disk in each file) dedicated to this purpose. Workers will also acknowledge that present-day systems commonly detect transducer positioning (servo) signals according to amplitude-modulation techniques (i.e., by variations in the amplitude of position-indicating recorded magnetic transitions, or "servo-bits"), and that this is less than optimal. For instance, the amplitude-sensitive transducers typically required are all too subject to "noise". Since erroneous amplitude variations can result from many common sources, such "noise" makes the servo systems based on this approach subject to serious error. An example of this approach is found in U.S. Pat. No. 3,864,740 to Sordello, et al. and in U.S. Pat. No. 3,185,972 to Sippel and in U.S. Pat. No. 3,614,756 to McIntoch, et al.
The Sordello patent in particular will indicate the lengths to which workers have gone to try to compensate for the difficulties arising from amplitude sensing. That is, Sordello will be seen to represent a "track following" method of detecting transducer position wherein prerecorded tracks are positioned on a recording medium so as to facilitate the direct detection of transducer position relative to the medium. A related U.S. Pat. No. 3,404,392 to Sordello discloses a track-following servo system. In this system a special pair of servo tracks is laid down on either side of each data track, the servo-bits in one of these servo tracks being recorded at one signal frequency and those of the other at a second frequency, the corresponding servo-output signals being frequency-separated by electronic filtering means which generated a summed servo signal. With such a system it is imperative that the frequencies of the two servo tracks differ sufficient to permit effective filtering and signal separation since the associated detecting transducer was "reading" both tracks simultaneously.
Conversely the first-named Sordello U.S. Pat. No. 3,864,740 postulates a pair of adjacent servo tracks wherein equal-amplitude servo signals are impressed, the servo tracks being prerecorded on the medium with a relatively inconsequential frequency difference; then, upon detection, the resultant servo signals are frequency-multiplied. That is, a pair of servo signal detection means are provided to modulate (multiply) the transducer output with modulating signals at the frequency of the first and second servo track waveforms and thereby generate a summed servo-output. This output represented the frequency difference between the original servo signal and respective modulating signal. By detecting the magnitude of this output (difference) signal, servo signals are generated for regulating servo positioning means.
Such a system will servo the transducer into registry with a selected data track. Workers will realize that the servo tracks flanking each data track represent a single continuous linear recording at two different encoded frequencies. Thus, if a single transducer is arranged to simultaneously read a data track and the flanking servo tracks--all together--and if means are provided to filter the servo information from the data signals, and then compare the two servo signals: then, one may develop a "position-error" signal and apply it to an actuator-servo unit to reposition the transducer.
However, such a system has the inherent disadvantage that the data and servo-bits must be separately recorded and at widely-spaced frequencies. Also, the servo frequencies cannot be harmonic of one another lest there be any harmful interaction between the (data and the servo) outputs. Another serious disadvantage, is that such a magnetic transducer will have a different transfer function for the data bits (frequency) then it has for the servo-bits (frequencies); and this can introduce further error.
Similarly, in the cited McIntoch patent a transducer positioning system is taught which comprises a magnetic disk with servo tracks and data tracks, with the magnetic domains of the servo tracks oriented relatively orthogonal to those of the data tracks. A transducer is provided to generate two outputs--a "data output" representing the rate of intensity change of the magnetic data domain and a "servo output" representing a function of the absolute magnitude of magnetic field represented by the magnetic servo domains. A flux-sensing portion of the transducer detected this servo output and thus indicated the transducer position relative to the data track, presenting an "error signal" to a servo positioning means.
One feature of such a servo system is that it provides a head repositioning-(or servo-error-) signal which is independent of medium movement relative to the transducer--that is, the acceleration or deceleration of the medium will not effect transducer response--evidently because the flux-sensing means will provide the prescribed output independent of whether the medium is moving at different speeds or is motionless. Also such a servo system provides for orthogonal isolation between (the magnetic influence of adjacent) servo-bits and data bits so recorded. This invention provides the same advantages while eliminating the need for a separate servo track. Other approaches are known which involve separate servo tracks (e.g., U.S. Pat. No. 4,007,493 to Behr, et al.; U.S. Pat. No. 3,964,094 to Hart); where, by contrast, systems according to the invention do not.
Workers are aware of present-day magnetic recording systems that use prerecorded servo tracks (e.g., see U.S. Pat. No. 3,903,545 to Beecroft, et al.; No. 2,938,962 to Konins, et al.; No. 3,404,392, to Sordello; and No. 3,185,972 to Sippel). One implementation involves a stacked multi-gap transducer adapted to register an intermediate head-gap over a "selected" data track while using a pair of flanking gaps to read servo-bits from a pair of servo tracks flanking each data track.